We previously found that TSP1 and CD47 null mice are remarkably resistant to soft tissue irradiation. This protection was reproduced in isolated vascular cells from the null mice, indicated that radioprotection is cell-autonomous. Importantly, tumors growing in the null mice were not protected, suggesting that therapeutic blocking of this pathway selectively protects normal but not tumor tissue. Based on these findings, we developed strategies to protect normal tissues from radiation damage using CD47 or thrombospondin-1 antibodies, a CD47-binding peptide, or antisense suppression of CD47. A morpholino oligonucleotide targeting CD47 confers radioresistance to human endothelial cells in vitro and protects soft tissue, bone marrow, and tumor-associated leukocytes in irradiated mice. In contrast, CD47 suppression in mice bearing melanoma or squamous lung tumors prior to irradiation result in 89% and 71% smaller tumors, respectively. Thus, inhibiting CD47 signaling maintains the viability of normal tissues following irradiation while increasing the radiosensitivity of tumors. Soluble guanylate cyclase (sGC) is the signal transduction enzyme most responsible for mediating the effects of nitric oxide (NO). Recently, NO-independent small molecule activators of sGC have been developed that have promising clinical activities. We have shown that the secreted matrix protein thrombospondin-1 binds to CD47 and potently inhibits NO stimulation of sGC in endothelial and vascular smooth muscle cells and platelets. Here we show that thrombospondin-1 signaling via CD47 inhibits sGC activation by NO-independent sGC activating small molecules. Vascular smooth muscle cells and washed human platelets were pretreated with thrombospondin-1 (2.2 nM) in the presence of heme-dependent sGC activators (YC-1, BAY 41-2272), and a heme-independent activator (meso-porphyrin IX), and cGMP levels were measured. The effect of sGC activators on platelet aggregation and contraction of smooth muscle cells embedded in collagen gels was also assayed in the presence and absence of thrombospondin-1. Thrombospondin-1 inhibited sGC activator-dependent increase in cGMP in vascular smooth muscle cells and platelets. Thrombospondin-1 pretreatment also inhibited the ability of these agents to delay thrombin-induced platelet aggregation. Thrombospondin-1 pretreatment reduced the ability of sGC activating agents to abrogate smooth muscle cell contraction in vitro. This work demonstrates that thrombospondin-1 is a universal inhibitor of sGC, blocking both heme-dependent and heme-independent activation. These data coupled with the reported increases in thrombospondin-1 with age, diabetes, ischemia/reperfusion, and atherosclerosis implies that the therapeutic potential of all drugs that activate sGC could be compromised in disease states where thrombospondin-1/CD47 signaling is elevated. Nitric oxide has prosurvival effects that can limit ischemia-reperfusion injuries. However, thrombospondin-1 is induced following ischemia-reperfusion injury and limits nitric oxide signaling by engaging its cell surface receptor CD47. We examined whether postinjury blocking of this inhibitory signal could protect from ischemia-reperfusion injury in a rat flap model. Flaps were created in rats based solely on the deep inferior epigastric vessels. Microvascular clamps were used to create 45 minutes of ischemia time to the flaps. The flaps were then treated using a monoclonal antibody to CD47 or an isotype-matched control antibody 5 or 30 minutes after clamp removal. Twenty-four or 72 hours postoperatively, the necrotic area of the flap was determined, and serum, deep inferior epigastric vessels, and flaps were harvested for analysis. Treatment with a CD47 antibody 5 minutes after reperfusion significantly reduces flap necrosis compared with immunoglobulin G1 control (9 percent versus 43 percent;p less than 0.01). The protective effect is even more dramatic when treatment is delayed until 30 minutes after reperfusion (10 percent versus 88 percent for control;p less than 0.01). Markers of neutrophil and endothelial cell activation along with total leukocytes are reduced in CD47 antibody-treated flaps, as are tissue malondialdehyde levels. Levels of cyclic guanosine monophosphate are elevated 72 hours postoperatively in the CD47 antibody-treated deep inferior epigastric vessels versus the control flaps. These studies indicate that therapies targeting the thrombospondin-1 receptor CD47 can increase tissue survival in ischemia-reperfusion injuries. The ability to protect when given after ischemia-reperfusion injury enables a broader clinical applicability.